The modern analog cellular system for mobile wireless duplex voice transmission is called "Advanced Mobile Phone Service" (AMPS). The AMPS cellular network uses the FCC assigned carrier frequency range of 800 to 900 MHz. Automobile-mounted cellular units transmit voice signals to a cellular base station within a given cell using up to one watt of power. Hand-held cellular units using battery power supplies transmit voice signals to a cellular base station within a given cell using up to one quarter watt of transmission power.
The analog human voice was the signal that the AMPS system was first designed to communicate. The AMPS system was optimized for carrying as many analog voice signals within a given bandwidth of a channel as possible. Mobility of the cellular telephone using low power mobile units, FM modulation, and the higher carrier frequency range (800 MHz-900 MHz) is achieved through a cellular arrangement of base stations whereby a user's signal is handed off to the next cell site as he or she moves into a different cell area. This cellular hand-off can cause a temporary loss in transmission or reception. However, temporarily losing a voice signal is not critical because a user knows when there is a signal loss and the voice information can be retransmitted. However, signal loss, even though temporary, poses special problems for transmission of digital data. Some other AMPS cellular problems causing loss in voice signals are drops in signal strength, reflections, Rayleigh fading, and cellular dead spots.
The availability of portable computers naturally led to the desire to conduct wireless transmission of digital data from a remote location. Presently, the AMPS voice cellular system is being used to transmit digital data in the form of circuit-switched cellular data across AMPS carrier channels. Raw (baseband) digital data must be converted so that it can be transmitted and received across the analog AMPS system. One disadvantage to using the AMPS system for data transmission is that a narrow channel bandwidth and errors in transmission limit the baud rate for transmitting and receiving the digital data. As previously stated, loss of raw digital data may be caused by other problems in the AMPS mobile cellular system.
Heretofore, providing efficient wireless communication of both voice and data signals in an integrated package has been difficult. Furthermore, it is difficult to integrate the features of AMPS voice transmission with applications such as data transmission, electronic mail, duplex paging, as well as the provision of a circuit-switched cellular data interface such as a wireless fax-modem, into a single hand-held battery operated wireless unit. This has been accomplished in part by the Cellular Digital Packet Data (CDPD) system described in the CDPD specification, Version 1.1, incorporated herein by reference as background material. The CDPD communication system shares the same carrier frequencies assigned to the AMPS channels as described in Part 405, Version 1.1 of the CDPD specification.
The base unit or mobile data base station (MDBS 1, as illustrated in FIG. 1), of a CDPD system utilizes a channel within an AMPS cell to establish a link and communicate to a user's mobile end system. The MDBS may use other frequencies outside of AMPS that are made available to it by service providers. The mobile end system (M-ES 2) is a portable computer, hand-set or other portable electronic device containing a subscriber communication unit. The MDBS serves as a communication link between the user of the subscriber station M-ES 2 and a service provider's network of wire lines, microwave links, satellite links, AMPS cellular links, or other CDPD links (such as mobile data intermediate system MD-IS 3 and intermediate systems 4, 5, 6) to convey data to another mobile end system, computer network, or non-mobile or fixed end-user system (F-ES 7, 8).
The CDPD network is designed to operate as an extension of existing communication networks, such as AMPS networks and the Internet network. From the mobile subscriber's perspective, the CDPD network is simply a wireless mobile extension of traditional networks. The CDPD network shares the transmission facilities of existing AMPS networks and provides a non-intrusive, packet-switched data service that does not impact AMPS service. In effect, the CDPD network is entirely transparent to the AMPS network, which is "unaware" of the CDPD function.
The CDPD system employs connectionless network services (CLNS) in which the network routes each data packet individually based on the destination address carried in the packet and knowledge of current network topology. The packetized nature of the data transmissions from an M-ES 2 allows many CDPD users to share a common channel, accessing the channel only when they have data to send and otherwise leaving it available to other CDPD users. The multiple access nature of the system makes it possible to provide substantial CDPD coverage to many users simultaneously with the installation of only one CDPD station in a given sector (transmitting range and area of a standard AMPS base station transceiver).
The airlink interface portion of the CDPD network consists of a set of cells. A cell is defined by the geographical boundaries within the RF transmission range from a fixed transmission site such as MDBS 1, which can be received at acceptable levels of signal strength by mobile subscribers such as M-ES 2. The transmitter supporting the cell may be located centrally within the cell, with transmission being carried out via an omni-directional antenna, or the transmitter located at the edge of a cell and transmitted via a directional antenna. This second type of cell is also referred to as a sector. In typical configurations, the transmitters for several sectors are co-located. The area served by a set of cells has some area overlap so that a roaming mobile end system can maintain continuous service by switching from one cell to an adjacent cell in a manner roughly analogous to the standard hand-off in an AMPS system. The two cells are considered to be adjacent if an M-ES can maintain continuous service by switching from one cell to the other. This switching process, called cell transfer, is done independently of normal AMPS hand-off procedures.
In FIG. 1, the interface (A) between the mobile end system 2 and the MDBS 1 is an "air interface" constituted by a radio frequency link using standard AMPS frequencies. The MDBS 1 is connected to other mobile data base stations through a mobile data intermediate system (MD-IS) 3. A number of mobile data base stations can be under the control of a single mobile data intermediate system. The mobile data intermediate systems are connected to each other through intermediate systems such as 4 and 5 in FIG. 1.
The intermediate systems are constituted by at least one node connected to more than one sub-network (such as intermediate system MD-IS 3). The intermediate system has a primary role of forwarding data from one sub-network to another. The mobile data MD-IS 3 performs data packet routing based on knowledge of the current location of each mobile end system within the range of the mobile data base stations under the control of the MD-IS. The MD-IS is the only network entity that is "aware" of the location of any of the mobile end systems. However, under some circumstances (as defined by the CDPD specification, Version 1.1 incorporated herein as background material), particular mobile data base stations will keep track of behavior of specific subscriber stations. A CDPD-specific Mobile Network Location Protocol (MNLP) is operated between each MD-IS (through the intermediate system) to exchange location information regarding the mobile end systems.
The overall CDPD network is controlled by a network management system (NMS) 10 having an interface with at least one mobile data intermediate system 3. Using a special protocol, programming instructions can be transmitted from the NMS 10 through the MD-IS 3 to any number of mobile data base stations under proper conditions.
Such programming instructions can be used to convey useful network data to the MDBS, as well as configuring the operation of an MDBS with respect to such critical features as maintaining channel queues. The NMS also controls other CDPD system characteristics such as the timing of paging messages to coincide with the non-dormant periods of the M-ES hand-sets. One advantage of the subject CDPD system is the capability of providing operating instructions to mobile data base stations from the NMS 10 through an MD-IS 3, or by a direct connection to the MDBS as is outlined in the detailed description of the MDBS architecture found in the CDPD specification, Version 1.1, Parts 402 and 403.
FIG. 2 depicts a comparison between the CDPD network illustrated in FIG. 1 and the standard AMPS network. The MDBS 1 is the CDPD equivalent to an AMPS base station 21. Both serve as links to mobile users, 2, 2', and 2" for the CDPD system and 22, 22" and 22" for AMPS users. 10 Both AMPS and CDPD functions can be handled by the same hand-set or end system equipment. Also, the MDBS 1 is preferably located with the AMPS base station 21 as will be explained in greater detail later.
The MD-IS 3 which acts as a local controller for the CDPD mobile data base stations connected thereto is equivalent to the mobile telephone switch office (MTSO) 23 used to control a plurality of AMPS base stations 21, 21' and 21". In the AMPS system, the MTSO 23 can be connected to the various base stations 21, 21', 21" by way of communication links, either over dedicated landlines or through a Public Switched Telephone Network (PSTN). Likewise, the connection between MD-IS 3 and the various mobile data base stations 1, 1', 1" controlled thereby is made in the same manner. However, different signaling protocols are used than those found in the AMPS system.
In comparison to AMPS, the infra-structure requirements of CDPD are very small. The CDPD base station equipment is located (preferably) at a cellular carrier's cell site along side existing AMPS base station cellular equipment. The multiple access nature of the CDPD system makes it possible to provide substantial CDPD coverage to many users simultaneously with the installation of only one CDPD radio in a given sector. This multiple access is the result of a mobile end-system accessing the CDPD channel only when there is data to be sent.
The AMPS base station and the MDBS can use the same RF equipment if both are co-located. By contrast, the MTSO of the AMPS system and the MD-IS of the CDPD system do not have to be co-located in order to share RF links. In the AMPS system, the MTSO 23 has the responsibility of connecting the AMPS base station and the mobile station to another party through a PSTN 24. The intermediate system 4 of the CDPD corresponds to the use of the PSTN by the AMPS system. Like the AMPS system, the CDPD system must also use the public switch telephone network or another landline network for completing calls to remote parties or systems via a phone system terminal network 28. However, the CDPD system employs a different protocol than that used by the AMPS system for completing calls over a PSTN.
In general, the CDPD system illustrated in FIG. 1 operates to provide service to manage data communications to subscribers over a wide geographic range. When a mobile end system is located in its home area, data packets are routed directly to and from it by the home MD-IS via the home MDBS. The route via which data packets are forwarded to and from a mobile end system or subscriber station changes when the mobile end system moves out of its home area.
The MDBS maintains zero or more (up to the MDBS transmission capability) channel streams across the airlink interface, as directed by the MD-IS controlling that MDBS. The MDBS instructs all subscriber units to change channels when necessary such as when an AMPS communication is detected on the CDPD channel. Each subscriber unit's terminal stream is carried on one channel stream at a time, normally selected by the mobile subscriber, preferably based upon data received from the MDBS regarding optimum channels for CDPD use. The forward and reverse traffic in a given cell (the terminal stream of the MDBS) is carried on a single DS0 trunk, between the MDBS and the MD-IS. Communication between the MDBS and the MD-IS over the DS0 trunk follows standard formats such as T1.
Within the CDPD network, digital data is transmitted between the MDBS and the M-ES using Gaussian Minimum Shift Keying (GMSK) modulation. Transmission from the base station to the subscriber station M-ES are continuous. Transmissions from subscriber station M-ES to the MDBS use a burst mode in which subscriber station M-ES only accesses a channel when it has data to send and the channel is not being used by other mobile subscriber stations. This allows a multiple mobile subscriber stations to share a single channel, and for data transmission characterized by intermittent transactions of relatively small amounts of data, thereby greatly reducing the connection time compared to that when sending digital data over conventional circuit-switched cellular modems.
Unlike the signaling schemes used in conventional cellular modems, which have been chosen based on the need to operate within the constraints of the existing voice signaling system, the GMSK modulation technique used for CDPD communication was explicitly selected with the intent of obtaining both very high bit transmission rates and very good error performance in cellular channels. The fact that the choice of modulation was not constrained by a pre-existing signal structure allows CDPD systems to achieve substantially greater instantaneous bit rates at very low received signal levels when compared to those of conventional cellular modems. This means that CDPD communication systems will provide reliable, high speed data transmission in many areas where signal quality is inadequate for good cellular modem performance. Presently the raw (baseband) digital data being transferred across CDPD include electronic mail messages, digital fax data, or digital data representing a network connection such that files may be transferred as if currently connected to a local area network.
The mobile data intermediate system MD-IS 3 handles the routing of packets for all visiting mobile end systems in its serving area. Two services are performed by the MD-IS: a registration service maintaining an information base of each M-ES currently registered in a particular serving location; and a re-address service, decapsulating forwarded packets and routing them to the correct cell. The serving MD-IS also administers authentication, authorization and accounting services for the network support service applications.
A CDPD communication system can operate with dedicated channels set aside from the pool of cellular voice channels and reserved for CDPD use. In the alternative, in a more typical mode of operation, the CDPD communication system can use idle time on channels that may also be used by AMPS communications. In this second case, the mobile data base station may perform "RF sniffing" to determine which channels are available and to detect the onset of voice traffic on the channel currently being used for CDPD communication. If an AMPS cellular unit begins transmitting on a channel occupied by a CDPD communication, the CDPD unit ceases transmitting on that channel and switches to another available channel (a process called "channel hopping") or if no other channel is available, ceases transmission until a channel becomes available for CDPD use.
Although the CDPD system shares existing AMPS radio frequency channels, as stated above, AMPS calls are given first priority, and they are always able to pre-empt the use of any channel being used by CDPD. However, the cellular service provider may opt to dedicate one or more channels to CDPD usage. In this case, AMPS calls will never attempt to pre-empt the channels dedicated to CDPD use.
In a normal operation of the MDBS carrying out channel hopping, the MDBS functions the monitor activity on AMPS channels. The MDBS maintains a list of the status (occupied voice or unused) for each channel available for CDPD use at the cell. The MDBS selects a channel for CDPD use from the unused channels in the list based on a combination of criteria (not specified in the CDPD standard). These could include such considerations as: the likelihood that the channel will be required by the voice system in the near future; the amount of interference present on the channel; the amount of interference that the CDPD communication is likely to cause to other voice users in different cells, or on other sectors; or, other factors. The MDBS transmits a list of all channels available for CDPD use (whether currently occupied by a voice communication or not) to the subscriber stations. The MDBS may execute a channel hop before the channel is pre-empted by AMPS communication if the MDBS determines that another channel is more suitable. In such a case, the MDBS sends a message to the subscriber stations commanding them to change to the specific channel selected, and then the MDBS executes the hop. This sort of hop is much more orderly and efficient since the subscriber stations do not have to search for the next channel.
If the present CDPD channel is pre-empted by AMPS communication, the MDBS selects another channel from those unused by AMPS communications and immediately hops to it without informing the subscriber station. The subscriber station then determines that the CDPD signal is no longer present on the current channel and searches the other channels in the list to determine the channel (if any) to which the CDPD communication has hopped.
The CDPD system has the capability of easily interfacing with the existing AMPS system and sharing some front-end equipment with it. To take advantage of this capability, the MDBS must have the capability of physically interfacing with existing AMPS base stations. Consequently, the MDBS should be small, non-obtrusive, and easily accessible without interrupting existing AMPS equipment. The MDBS has to be configured so as to easily be connectable to equipment outside the MDBS normally shared with the AMPS system. This external equipment found in the AMPS base station includes an antenna system, RF power amplifiers (in particular, linear amplifiers can be shared with existing AMPS), RF multicouplers, power splitters, duplexers, and, optional equipment. Since the MDBS shares the environment of the AMPS base station, the MDBS should not constitute a substantial additional burden upon such support systems as environmental control and maintenance. Thus, the MDBS must be compact and flexible, constituting only those elements necessary for carrying out the MDBS functions necessary at that cell site.
The use of an effective CDPD system has brought about a problem in that a subscriber station must attempt to monitor for incoming calls on both CDPD and AMPS communication systems. If the subscriber station adheres to the timing of the CDPD system, it is likely that incoming AMPS communications will be ignored, despite the pre-emption given to AMPS communications over CDPD communications. And while priority can be given to monitoring for AMPS communications, it is probable that CDPD communications directed to a subscriber station will be lost despite the fact that the CDPD system can buffer incoming paging signals for sleeping CDPD subscriber stations. Existing CDPD communication systems and existing AMPS communication systems fail to provide efficient monitoring of both modes of communications to prevent loss of incoming calls.